In electrophotographic copier machines of the transfer type it is customary to utilize a support surface such as a rotating drum or a belt for mounting photoreceptive material upon which an image of the original document is produced. After producing the image it is developed and transferred to copy paper. The process requires the charging of the photoreceptive material to a relatively high voltage level, before exposing it to light rays reflected from the original document. The photoreceptive material is thereby discharged in an amount dependent upon the intensity of the light rays received and thereby caused to bear an electrostatic image of the original. Development is typically through the use of a black powdery substance called toner which is deposited on the undischarged portions in greater amount than the discharged portions. It is the black powdery toner which is transferred to copy paper causing the copy paper to bear an image of the original. Toner is then fused to the copy paper to produce a finished copy.
The uniform electrostatic charge placed upon the photoreceptor prior to exposing it to light rays reflected from the original is typically produced by a charge corona generator. The charge corona generator is comprised of the requisite number of emission wires raised to high voltage levels so as to ionize the air surrounding the emission wire and create a flow of charge to the photoreceptive surface. Such corona generators are well known in the art and are exemplified by U.S. Pat. No. 3,736,424.
The most popular developer mechanism in recent years has been the magnetic brush developer which is exemplified by U.S. Pat. No. 3,999,514. This type of developer is essentially comprised of a hollow rotating conductive shell surrounding permanent magnets inside the shell. The permanent magnets act to attract magnetizable materials to the surface of the rotating shell in order to carry the magnetizable materials from a reservoir to a development zone. In some cases the magnetizable material may be toner and in other cases it may be desirable to use small steel carrier beads which are coated with non-magnetic toner. In that manner the steel beads are attracted to the rotating shell by the permanent magnets within that shell and rotated on the surface of the shell from a reservoir to a development zone. At the development zone the toner is dislodged from the steel carrier beads and deposited upon the image of the original document. The steel carrier beads and the extra toner then fall from the development zone back into the reservoir.
The toner particles carry a natural electrostatic charge, i.e., a triboelectric charge, which may for example, be positive. The steel bead may be coated with a material such as "Teflon" which carries a negative triboelectric charge. Consequently, the positive toner is attracted to the negative carrier bead so that when the carrier bead is attracted magnetically to the surface of the magnetic brush roll, it is carried to the development zone. Through agitation at the development zone the toner is dislodged from the carrier bead and attracted to the surface of the photoreceptive material which, if the toner is positive, must be a highly negative surface. Thus, in the arrangement described, the charge corona must be a negative corona depositing a negative charge on the photoreceptive material. It should be noted that the charge structure can be reversed depending upon the type of photoreceptive material used, i.e., the charge corona could deposit a positive charge and the toner material could carry a negative triboelectric charge.
It has been found that despite the electrostatic attraction of toner particles to the oppositely-charged photoreceptive material, there is nevertheless a tendency for some of the toner particles to escape from the photoreceptive surface and move out into the body of the machine. It is believed that a major portion of toner contamination results from the transfer operation where toner particles are loosened and removed from the photoreceptive material and may not be captured on the surface of the copy paper. Much of that stray toner is captured and entrained in a boundary layer of air which rotates with the rotating photoreceptive material. While it is always unfortunate to contaminate machinery with a black, powdery carbon-like substance such as toner, it is particularly troublesome to create contamination of negative coronas. Suppose, for example, that a negative charge corona is being used. If positively-charged toner is deposited in the charge corona housing it is likely to strike the negative emission wire and contaminate that wire. In the course of time, deposits on the emission wires will create streaking on produced copies due to the formation of nodes or hot spots caused by the contamination. Even when positive charge coronas are used, negative preclean coronas must be used and a similar problem results within the preclean corona. Furthermore, it has been found that low pressure areas can exist within corona housings causing the formation of a vortex within the housing and a disturbance of the boundary layer bringing toner contamination into the corona. It is, therefore, a general object of this invention to reduce contamination within corona generators of an electrophotographic copier machine by reducing the amount of toner which may find its way into the corona housings.
In addition to toner contamination, occasionally a steel carrier bead will escape from the magnetic brush developer and be carried away on the photoreceptive surface. These stray steel beads are held on the rotating surface by "fringe fields" set up at the boundary of a highly charged area of the photoreceptor and an area which has been discharged. If anything interferes with the fringe field, the stray carrier bead is loosened from the photoreceptor and can escape into the machine. If these beads are swept into corona housings, corona arcing might occur. It is therefore a general object of this invention to remove stray carrier beads from the photoreceptive surface and deposit them in an area of little influence.